This invention relates to pump constructions for small hand-held dispensers and the like, and more particularly to pumps wherein a hollow plunger slidably carries at its lower end a hollow piston which is movable on the plunger between high and low positions at the time of the initial plunger movement. A pump construction of this type is illustrated and described in U.S. Pat. No. 3,257,961, issued to Schlenker and dated June 28, 1966. In this patented pump construction the hollow plunger has press-fitted into it the shank of a valve stem provided with an enlargement or head at its lower end, which is cooperable with an annular valve seat at the underside of the piston. Venting of the pump construction is accomplished by a special configuration of the plunger, piston and valve head which provides a somewhat devious path for the air entering the container to replace the liquid being dispensed therefrom. The parts involved in the venting co-act in such a manner that the free inflow of air into the container, to replace the discharged liquid, is not always insured. Moreover, the parts are so constructed and function in such a manner that critical dimensions and tolerances are required, resulting in an unnecessarily high fabrication and assembly cost.
Other prior pump constructions which have a more direct path for the influx of air, involve complicated bushing, cylinder and piston structures which can easily malfunction, causing undesired leakage of the container contents during shipping and storage, while still having the undesirable critical tolerances and high fabrication cost.